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The ±1100 kV Zhundong–Wannan ultra high-voltage direct current (UHVDC) transmission project is under construction in China, which has the highest operating voltage in the world. The length of the UHVDC overhead transmission line is greater than 3000 km. The electromagnetic transient process caused by grounding faults might have significantly impact on the overvoltage level of DC transmission line, which is crucial for the external insulation configuration of the overhead transmission line. This paper presents a detailed electromagnetic transient simulation model for the UHVDC transmission system using an open-loop extinction angle control principle. The simulation results demonstrate that the single pole grounding fault into a bipolar system causes the most serious damage. The maximum overvoltage is 1.628 p.u. and the 2% statistic overvoltage is 1.635 p.u., while the reference value is 1122 kV. The influence of DC filters for line overvoltage is also analysed here. Furthermore, results show that adding a no gap line arrester at the midpoint of transmission line can suppress line overvoltage, while the maximum overvoltage reduces by 0.138 p.u. The results obtained provides a reference for the optimisation of the external insulation configuration. [ABSTRACT FROM AUTHOR]

In this paper, the variation of the development of positive first corona space charge in long air gaps is investigated. A 1-m rod-plane air gap under different rising rates dU/dt of impulse voltages is reported. The current signals and still photographs are recorded during the first corona development. The relationship between the first corona charge Qi and the inception voltage Ui is presented. The physical mechanism of the influence of the first corona is provided. With the experiments and the single branch propagation model, a semi-empirical calcultion method for positive first corona charge under impulse in long air gaps is obtained through the calculation of the number of branches Nb. Using this method, the variations between Qi and Ui for different dU/dt and electric field distributions are calculated and compared with the reported experimental results. Reasonable agreement between the calculated and experimental results is obtained. It shows that this calculation method is suitable for predicting the positive first corona charge Qi and the discharge current at a given dU/dt and Ui. [ABSTRACT FROM PUBLISHER]

In order to investigate the different features of the positive leader propagation in the rod–plane and inverted rod–plane gaps, the discharge experiments in a 3-m rod–plane gap and a 3-m inverted rod–plane gap were carried out under switching impulse voltage. The positive leader propagation process was recorded by using a high-speed CCD camera. Some aspects to describe the positive leader propagation characteristics of the rod–plane and inverted rod–plane gaps are obtained and compared in this paper, including the critical corona radius, the average continuous leader inception voltage, the axial leader velocity, and the final jump length. The possible reason resulting in those differences is also discussed here. [ABSTRACT FROM PUBLISHER]

A novel observation method is proposed to study the streamer-leader propagation processes of long air-gap positive discharges. Experiments of 3-12 m air-gap discharges have been conducted in this paper. The observed images could clearly and particularly describe the characteristics of the positive streamer-leader propagation processes. Some important phenomena of the propagation processes are indicated and discussed, such as the leader's velocity that varied with the time, the different lengths of inception leaders, the leader's branching phenomena, the continuous streamer-leader propagation processes simultaneously with the voltage's descent, and the upward leader's propagation processes. [ABSTRACT FROM AUTHOR]

• It is the first application of MPC in MMC-UPFC to handle the unbalanced fault. The current reference of the MPC algorithm for the shunt MMC is obtained in synchronous dq coordinate frame. Previous work has not involved this procedure. • The voltage constraint is considered in the MPC for series MMC. It enables the MMC to operate with the maximum output voltage of the converter by modifying the power reference values in the cost function. It achieves the suppression of the unbalanced current and also makes full use of the capacity of UPFC by operating it on the voltage boundary. This paper proposes a model predictive control (MPC) method to tackle the power flow control of the modular multilevel converter based unified power flow controller (MMC-UPFC) under unbalanced grid conditions. The predictive models corresponding to the discrete-time models of MMC-UPFC are derived. For the shunt MMC, the d-axis and q-axis current references for the MPC are deduced to maintain the dc-link voltage constant according to the energy balance on the dc-link side. The series MMC utilizes a cost function with the voltage constraints to suppress the faulted current and make the power flow controllable, and thus avoid the spread of the fault. Moreover, it makes full use of the capacity of MMC by operating it on the voltage boundary defined by the maximum output voltage of the converter. The model is established using the parameters from a 500 kV MMC-UPFC practical project in Jiangsu, China. Simulation results demonstrate a favorable response of the MMC-UPFC based on the proposed MPC strategy under different unbalanced grid conditions. [ABSTRACT FROM AUTHOR]